Coded and uncoded CPM schemes over rings of integers for AWGN andRayleigh fading channels

Coded and uncoded modulation schemes based on continuous phase modulation (CPM) over rings of integers are investigated. A baseband modulator utilising a decomposed model of CPM is used at the transmitter and a Viterbi soft-decision decoder is employed at the receiver. The performances of a number of partial-response CPM schemes are investigated when operating over an additive white Gaussian noise (AWGN) channel and Rayleigh flat fading (RFF) channel. The simulation results obtained are shown to compare favourably with theoretical bounds     

基于GaN HEMT的13～17 GHz收发多功能芯片

基于GaN高电子迁移率晶体管(HEMT)工艺设计制作了一款收发(T/R)多功能芯片(MFC),主要用于射频前端收发系统.该芯片集成了单刀双掷(SPDT)开关用于选择接收通道或发射通道工作,芯片具有低噪声性能、高饱和输出功率和高功率附加效率等特点.芯片接收通道的LNA采用四级放大、单电源供电、电流复用结构,发射通道的功率放大器采用三级放大、末级四胞功率合成结构,选通SPDT开关采用两个并联器件完成.采用微波在片测试系统完成该芯片测试,测试结果表明,在13～ 17 GHz频段内,发射通道功率增益大于17.5 dB,输出功率大于12W,功率附加效率大于27％.接收通道小信号增益大于24 dB,噪声系数小于2.7 dB,1 dB压缩点输出功率大于9 dBm,输入/输出电压驻波比小于1.8∶1,芯片尺寸为3.70 mm×3.55 mm.     

Decoupled compensation of IQ imbalance in MIMO OFDM systems

The direct-conversion architecture is an attractive front-end design for multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. These systems are typically small in size and provide a good flexibility to support growing number of wireless standards. However, direct-conversion based OFDM systems are generally very sensitive to front-end component imperfections. These imperfections are unavoidable especially when cheaper components are used in the manufacturing process and can lead to radio frequency (RF) impairments such as in-phase/quadrature-phase (IQ) imbalance. These RF impairments can result in a severe performance degradation. In this paper, we propose training based efficient compensation schemes for MIMO OFDM systems impaired with transmitter and receiver frequency selective IQ imbalance. The proposed schemes can decouple the compensation of the transmitter and receiver IQ imbalance from the compensation of the channel distortion. It is shown that the proposed schemes result in an overall lower training overhead and a lower computational requirement as compared to a joint estimation/compensation of IQ imbalance and the channel distortion.     

Improved auxiliary particle filter‐based synchronization of chaotic Colpitts circuit and its application to secure communication

In this paper, we propose a synchronization scheme based on an improved auxiliary particle filter (IAPF) for chaotic Colpitts circuit and conduct an experimental study on the synchronization performance with application to secure communications. Specifically, with the synchronization scheme, when the chaotic signals generated by an analog Colpitts circuit are transmitted through a nonideal channel, the distorted signals are processed digitally by the novelly designed IAPF at the receiver, in order to obtain the synchronized signals of the transmitter circuit. Experimental results indicate that synchronization can be achieved over both the additive white Gaussian noise channel and the multipath fading channel with low signal‐to‐noise ratio, even if there exist severe circuit parameter mismatches between the transmitter and the receiver. Furthermore, a chaos‐masking secure communication system is constructed and verified over both the additive white Gaussian noise channel and the multipath fading channel, and the bit error rate is evaluated versus different signal‐to‐noise ratios and symbol periods. It is shown that the achievable bit error rate can reach the order of magnitude of 10 ^{− 4} without error correction coding techniques. In addition, security analysis demonstrates that the proposed chaotic secure communication system is resistant to the brute‐force attack. Copyright © 2013 John Wiley & Sons, Ltd.     

一种零中频发射反馈镜像及直流联合跟踪校正算法

针对零中频架构固有的镜像和直流失真问题,提出一种零中频发射反馈镜像和直流失真联合校正算法,通过不同的发射和反馈本振设置,从频率维度将发射镜像和反馈镜像、发射直流和反馈直流区分开,并使用业务信号进行在线训练,保证业务连续性,无需中断业务。上板测试表明,该算法能将发射和反馈镜像及直流稳定校正到底噪之下,明显改善了零中频架构的镜像和直流指标。     

SER Computation in M-QAM Systems with Phase Noise

This paper presents an exact closed-form expression for the symbol error rate of the square and rectangular quadrature amplitude modulation (QAM) constellations, under the assumption that the transmitted and/or received signals are corrupted by the phase noise fluctuation. Phase noise is one of the most important radio frequency (RF) imperfections which usually comes from the local oscillator (LO) at the transmitter and/or receiver. In this paper, the additive white Gaussian noise (AWGN) channel is assumed. Although AWGN is a simple channel, but our exact analysis of the symbol error rate in this paper can be led to the precise study of the real communication systems in the fading channel. Hence, an exact closed-form solution for the symbol error rate is derived here as a finite summation of the two-dimensional Q-function and verified by the simulation.     

An integrated framework for MC-CDMA reception in the presence of frequency offsets, phase noise, and fast fading

The combination of code-division multiple-access and multicarrier (MC) modulation has been proposed to develop high data-rate wireless communication systems. Due to the longer symbol duration in comparison with single-carrier systems, MC systems are more sensitive to various imperfections, including phase noise and frequency offsets due to local oscillators and Doppler spreading due to motion that results in temporal channel variations. The performance of current systems is significantly limited by these imperfections because they disperse the transmitted power in a particular subcarrier into adjacent subcarriers, thereby causing interference between the subcarriers at the receiver. We consider a single-user communication system and use a canonical model for the received signal that efficiently captures the effects of all impairments. The model uses Fourier basis functions that are fixed for all imperfections while the expansion coefficients depend on imperfections. Using the model, we introduce a receiver structure that implements the matched filter (MF), and hence, optimal. The MF is implemented through a Rake receiver in the frequency domain. The receiver fully compensates for frequency offsets as well as phase noise, thereby eliminating the performance loss due to these factors. Furthermore, in contrast to existing designs, it delivers improved performance under fast fading by exploiting Doppler diversity. Finally, the same integrated receiver structure works for all imperfections eliminating the need for devising a separate correction technique for each.     

Noncoherent Communication in Multiple-Antenna Systems: Receiver Design and Codebook Construction

In this paper, we address the problem of space-time codebook design for noncoherent communications in multiple-antenna wireless systems. In contrast with other approaches, the channel matrix is modeled as an unknown deterministic parameter at both the receiver and the transmitter, and the Gaussian observation noise is allowed to have an arbitrary correlation structure, known by the transmitter and the receiver. In order to handle the unknown deterministic space-time channel, a generalized likelihood ratio test (GLRT) receiver is considered. A new methodology for space-time codebook design under this noncoherent setup is proposed. It optimizes the probability of error of the GLRT receiver's detector in the high signal-to-noise ratio (SNR) regime by solving a high-dimensional nonlinear nonsmooth optimization problem in a two-step approach. First, a convex semidefinite programming (SDP) relaxation of the codebook design problem yields a rough estimate of the optimal codebook. This is then refined through a geodesic descent optimization algorithm that exploits the Riemannian geometry imposed by the power constraints on the space-time codewords. The results obtained through computer simulations illustrate the advantages of our method. For the specific case of spatio-temporal white observation noise, our codebook constructions replicate the performance of state-of-the-art known solutions. The main point here is that our methodology permits extending the codebook construction to any given correlated noise environment. The simulation results show good performance of these new designed codes in colored noise setups.     

Effects of imperfect channel sensing and estimation on the performance of cognitive radio systems

In realistic scenarios of cognitive radio (CR) systems, imperfect channel sensing may occur due to false alarms and miss detections. Channel estimation between the secondary user transmitter and another secondary user receiver is another challenge in CR systems, especially for frequency‐selective fading channels. In this context, this paper presents a study of the effects of imperfect channel sensing and channel estimation on the performance of CR systems. In particular, different methods of channel estimation are analyzed under channel sensing imperfections. Initially, a CR system model with channel sensing errors is described. Then, the expectation maximization (EM) algorithm is implemented in order to learn the channel fading coefficients. By exploiting the pilot symbols and the detected symbols at the secondary user receiver, we can estimate the channel coefficients. We further compare the proposed EM estimation algorithm with different estimation algorithms such as the least squares (LS) and linear minimum mean square error (LMMSE). The expressions of channel estimates and mean squared errors (MSE) are determined, and their dependencies on channel sensing uncertainty are investigated. Finally, to reduce the complexity of EM algorithm, a sub‐optimal algorithm is also proposed. The obtained results show that the proposed sub‐optimal algorithm provides a comparable bit error rate (BER) performance with that of the optimal one yet with less computational complexity.     

Adaptive dual‐radio spectrum‐sensing scheme in cognitive radio networks

In this paper, a novel spectrum‐sensing scheme, called adaptive dual‐radio spectrum‐sensing scheme (ADRSS), is proposed for cognitive radio networks. In ADRSS, each secondary user (SU) is equipped with a dual radio. During the data transmission, with the received signal‐to‐noise ratio of primary user (PU) signal, the SU transmitter (SUT) and the SU receiver (SUR) are selected adaptively to sense one channel by one radio while communicating with each other by the other one. The sensing results of the SUR are sent to the SUT through feedback channels (e.g., ACK). After that, with the sensing results from the SUT or the SUR, the SUT can decide whether the channel switching should be carried out. The theoretical analysis and simulation results indicate that the normalized channel efficiency, defined as the expected ratio of time duration without interference to PUs in data transmission to the whole frame length, can be improved while satisfying the interference constraint to PUs. After that, an enhanced ADRSS is designed by integrating ADRSS with cooperative spectrum sensing, and the performance of ADRSS under imperfect feedback channel is also discussed. Copyright © 2011 John Wiley & Sons, Ltd.     

零中频数字通信发射机的射频指纹时域基带建模

射频指纹(RFF)来源于发射机电路设计的差异和生产过程中硬件电路的制造容差,是一种新兴的设备身份识别和认证技术。对射频指纹产生机理进行建模是深入研究射频指纹技术的基础。本文根据一种通用的零中频数字通信发射机结构分析了各环节对射频指纹的影响,并建立了对应的射频指纹时域基带模型。此外,归纳总结了一系列通信标准的若干重要时域参数的容差,并着重研究了LTE标准下,正交相移键控(QPSK)和十六进制正交振幅调制(16-QAM)两种典型调制方式的最大均方根误差向量幅度(RMS EVM)。最后,通过理论推导和Matlab仿真给出了直流偏置、同相/正交(I/Q)增益不平衡、I/Q正交偏移误差、I/Q滤波器偏差、振荡器相噪和功放非线性参数的上下界,并分析了各种射频指纹参数临界情形下星座图的变化,为射频指纹提取和识别技术的研究提供了合理的参数指导。     

Ultra-wideband transmitted reference systems

This paper derives optimal receiver structures for an ultra-wideband transmitted reference (UWB TR) system in multipath environments, based on the average likelihood ratio test (ALRT) with Rayleigh or lognormal path strength models. Several suboptimal receivers are obtained by either applying an approximation to the log-likelihood function without any specific channel statistical models or by approximating two ALRT optimal receiver structures. It is shown that the generalized likelihood ratio test optimal receiver is one of the suboptimal receiver structures in the ALRT sense. Average bit error probabilities of ALRT receivers are evaluated. Results show that ALRT optimal and suboptimal receivers derived from Rayleigh and lognormal models can perform equally well in each other's environments. This paper also investigates ad hoc cross-correlation receivers in detail, and discusses the equivalence between cross-correlation receivers and one theoretically derived ALRT suboptimal receiver. Results show that the noise /spl times/ noise term in a cross-correlation receiver can be modeled quite accurately by a Gaussian random variable when the noise time/spl times/bandwidth product is large, and cross-correlation receivers are suboptimal structures which have worse performance than ALRT receivers.     

Rake‐Based Cellular Radar Receiver Design for Moving Target Detection in Multipath Channel

In this paper, we design a rake‐based cellular radar receiver (CRR) scheme to detect a moving target located in a multipath environment. The modules of Doppler filter banks, threshold level test, and target detection module are newly introduced into the conventional rake receiver so that it can function as a radar system. The proposed CRR tests the Doppler‐shift frequency and signal‐to‐noise ratio of the received signal against predefined threshold levels to determine detection and then calculates target velocities and ranges. The system performance is evaluated in terms of detection probability and the maximum detection range under a Nakagami‐n channel that reflects the multipath environment.     

OFDM系统中基于压缩感知恢复由限幅和HPA产生的非线性失真研究

正交频分复用（Orthogonal Frequency Division Multiplexing,OFDM）系统的主要缺点之一就是有较高的峰均功率比（Peak to Average Power Ratio,PAPR）,降低了功率放大器（High Power Amplifier,HPA）的工作效率,同时HPA引入的非线性失真,恶化了系统的误比特率（Bite Error Rate,BER）性能.本文所提算法将限幅和HPA引入的非线性失真视为一个整体来考虑,利用与限幅噪声在时域上的近似稀疏性,对整个非线性过程进行建模.发送端通过限幅降低了OFDM信号的PAPR,在接收端,选取受噪声干扰小的可靠性观测向量,最小化信道噪声的影响,基于非线性模型计算得到的参数,利用压缩感知（Compressive Sensing,CS）算法能有效地恢复总的非线性失真信号,提升了系统的BER性能.     

Degradation of the MIMO channel capacity due to additional noises of amplifiers

It is theoretically well known that a multiple input multiple output (MIMO) system can offer higher channel capacity than a conventional wireless system. The MIMO capacity is increased as a function of signal‐to‐noise ratio (SNR) and a number of antenna elements at both the transmitter and receiver. To realize MIMO capability in practical use, the implementation of all electronic devices is necessary to be considered. Amplifiers become the most common devices assembled at the transmitter and receiver with the purpose of overcoming the power loss in the wireless channel. High power amplifier (HPA) and low noise amplifier (LNA) are the special types of amplifiers that are compulsory for the transmitter and receiver, respectively. Both devices have the same function to provide the reliable power gains while adding as little noise as possible. For a MIMO system, the channel capacity sensitively grows as a function of SNR. Therefore, the changes in SNR due to HPA and LNA definitely degrade the system capacity. In this paper, the degradation due to both devices is investigated by taking the gain and noise figure of these amplifiers into account. The new formula of MIMO channel capacity including amplifier characteristics is originally proposed. The results indicate the remarkable consideration on using amplifiers as they may degrade the capacity up to 26%. This can be the guideline for MIMO researchers to spare the capacity margin caused by the additional noises of amplifiers. Copyright © 2008 John Wiley & Sons, Ltd.     

Channel capacity and bit error rate optimization of the ultra‐wide bandwidth transmitted‐reference receiver

The channel capacity and the bit error rate of ultra‐wide bandwidth transmitted‐reference systems are optimized with respect to time delay between the reference and the adjacent data pulses. Approximate and theoretical expressions for the signal‐to‐interference‐plus‐noise ratio are derived. Numerical results show that optimizing the receiver can provide a significant capacity improvement of up to 2.2 bits/s/GHz and a bit error rate performance gain of up to 2 dB in effective signal‐to‐noise ratio. Copyright © 2011 John Wiley & Sons, Ltd.     

Kalman filter‐based channel estimation and ICI suppression for high‐mobility OFDM systems

The use of orthogonal frequency division multiplexing (OFDM) in frequency‐selective fading environments has been well explored. However, OFDM is more prone to time‐selective fading compared with single‐carrier systems. Rapid time variations destroy the subcarrier orthogonality and introduce inter‐carrier interference (ICI). Besides this, obtaining reliable channel estimates for receiver equalization is a non‐trivial task in rapidly fading systems. Our work addresses the problem of channel estimation and ICI suppression by viewing the system as a state‐space model. The Kalman filter is employed to estimate the channel; this is followed by a time‐domain ICI mitigation filter that maximizes the signal‐to‐interference plus noise ratio (SINR) at the receiver. This method is seen to provide good estimation performance apart from significant SINR gain with low training overhead. Suitable bounds on the performance of the system are described; bit error rate (BER) performance over a time‐invariant Rayleigh fading channel serves as the lower bound, whereas BER performance over a doubly selective system with ICI as the dominant impairment provides the upper bound. Copyright © 2008 John Wiley & Sons, Ltd.     

Channel estimation for nonlinear MIMO receiver with square envelope detection

Two practical channel estimation schemes, the moment‐based first‐and‐second moments and the simplified maximum likelihood estimators, are proposed for the MIMO/on–off keying system with square envelope detection applied for wireless sensor networks. Here, both the channel response and noise power are estimated simultaneously in comparison with other approaches in which the noise quantity is assumed to be known at the receiver. Hence, the developed estimators are more practical than those estimators without noise power estimation. Simulation results reveal that the system with both proposed schemes can achieve an excellent BER performance in a wide signal‐to‐noise ratio (SNR) range. More specifically, we observed that the simplified maximum likelihood estimator performed as well as the moment‐based first‐and‐second moments estimator for SNR greater than 7.5 dB, yet had much more decline at low SNRs. This study also investigated the effects of the numbers of receive antennas and transmit antennas on the system performance. Simulation results demonstrated that, at the BER of 10^?3, the 5 × 5 system had an improvement of 7 dB in SNR compared with the 3 × 3 system. Copyright © 2011 John Wiley & Sons, Ltd.     

Maximized achievable rate of SINR‐measurement‐based spectrum sharing with binary feedback

Motivated by the difficulty in measuring channel state information between heterogeneous primary and secondary systems, we propose a signal‐to‐interference‐noise ratio (SINR)‐based spectrum sharing policy. In the proposed spectrum sharing policy, a secondary user who does not cause SINR outage at the primary receiver is admitted to share the primary user's spectrum. In this article, we analyze the achievable rate of a secondary user and for whom the achievable rate is maximized in the SINR‐constrained spectrum sharing. The maximized achievable rate is determined by the ratio of the distance between the secondary transmitter and the primary receiver to the distance between the secondary transmitter and receiver, and it is proportional to the distance ratio. In conclusion, secondary links with large distance ratio can support high data rate applications so long as SINR constraint of the primary user is guaranteed. Copyright © 2011 John Wiley & Sons, Ltd.     

正交多载波降噪差分混沌键控通信系统

为解决多载波差分混沌移位键控(MC-DCSK)系统传输速率低和误码性能差的缺点,该文提出一种正交多载波降噪差分混沌移位键控(QMC-NR-DCSK)系统。在发送端,预定义载波用于发送参考信号,剩余M-1个不同中心频率的载波及其经正交调制技术后得到的频率相同但相位正交的载波都用于传输信息信号,此外,通过进一步引入Hilbert变换,将系统的频带利用率和传输速率提升为MC-DCSK系统的4倍。在接收端引入滑动平均滤波器的降噪操作降低了噪声的方差,从而改善了系统误码性能。推导了QMC-NR-DCSK系统在加性高斯白噪声(AWGN)信道和多径瑞利衰落(RFC)信道下的比特误码率公式并进行了仿真。仿真结果和理论分析表明:QMC-NR-DCSK系统能有效提升传输速率、带宽效率和误码性能,为该系统应用于多载波无线通信提供理论参考。